This invention relates to a device and method for measuring the physical properties of fluids, including liquids and gases, and in particular to a device and method for measuring physical properties of fluids using the interference of light waves.
The thermal conductivities of liquids are often measured because the thermal conductivities of liquids are required for various heat transfer analyses. These conductivities are commonly obtained with a transient hot-wire apparatus. In this technique a thin platinum wire, serving as both a heating element and a thermometer, is heated resistively with a current pulse of about one second duration. The thermal conductivity of the surrounding medium is determined from the temperature change of the wire as a function of time. In this type of analysis an approximate solution of the heat conduction equation is used, where the slope of the change in temperature versus the natural log of time curve is inversely proportional to the thermal conductivity of the medium. In applying this method, a number of corrections are necessary due to the finite diameter and finite thermal conductivity of the platinum wire. Additionally, a correction for the temperature dependence of the fluid properties is necessary. With these corrections incorporated into the analysis, the technique allows for thermal conductivity determinations with an accuracy of 0.2%.
Similarly, gas flow rates can be measured with a hot-wire anemometer which employs a fine wire mounted transversely to the gas flow. The wire is heated by an electrical current and the temperature rise, which depends inversely on the flow rate, is determined by the resistance change of the wire.
Because electrical sensing devices can be a source of explosion hazard when monitoring flammable liquids, fiber optic systems for measuring properties such as temperature, thermal conductivity, and flow rate are preferred. Such fiber optic systems for measuring temperature are well-known in the art. One such system is disclosed in Langeac U.S. Pat. No. 4,563,639, in which a probe is formed by winding an optical fiber in a generally solenoid shape. U.S. Pat. No. 4,621,929, issued to Phillips and entitled "Fibre Optic Thermal Anemometer," teaches a device for measuring the heat transfer coefficient of a sample by implanting in the sample an element with temperature-sensitive optical properties. The element is heated or cooled and the temperature difference between the element and the unheated sample and the rate of heating or cooling indicate the heat transfer coefficient of the sample. An optical fiber is connected to the element for transmitting radiation to and from the element. The fiber serves as a conduit rather than as the sensing element itself.
Optical fibers have been used previously as flow or velocity sensors. Pitt et al. have discussed several types of optical fiber flowmeters ranging from simple pulsed interruption methods with turbine impeller blades to vortex shedding and correlation techniques ("Optical Fiber Flowmeters", Proc. 2nd International Conference on Optical Fiber Sensors, Stuttgart, pp. 23-28, September 1984). Wide and Dandridge have described a fiber optic mass flowmeter for liquids which utilizes the Coriolis effect ("Fibre-Optic Coriolis Mass Flowmeter for Liquids," Electr. Lett. 24, 783, 1988). Fiber optic techniques have also been utilized in laser-Doppler anemometry. The Phillips patent discloses the use of his fiber optic device to measure flow. The temperature-sensitive element is heated with near infrared radiation which passes through the same optical fiber that optically communicates with the temperature sensor. The change in temperature produced by the infrared radiation is inversely dependent upon the flow rate of the fluid past the sensor. Again, the optical fiber is used to transmit the light to or from the sensor, and not as the sensing element.
U.S. Pat. No. 4,859,059, issued to Bobb et al. and entitled "Thermal Modulation of Light Beams" discloses controlling phase in an interferometer by electrically heating a gold-coated section of an optical fiber in the interferometer.